(1) Field of the Invention
The invention relates to a speech analysis system comprising means for receiving an input analog speech signal and means for determining at regularly recurring instants the mean value of the rectified speech signal in segments thereof preceding said instants, the mean values thus determined providing a measure for separating voiced speech segments from unvoiced speech segments.
(2) Description of the Prior Art
Such a speech analysis system is generally known in the art of vocoders. As an example reference may be made to Proceedings of the IEEE, Vol. 63, No. 4, April 1975, pp 662-677. It is mentioned therein, that an energy function of the speech signal, such as the afore mentioned mean value, which is also termed waveform intensity or average magnitude, is a good measure for separating voiced segments from unvoiced segments. However, it is found in practice that the voiced-unvoiced decision based hereon is unreliable for a range of values of the waveform intensity.
It has also been mentioned, that basically, a pitch detector is a device, which makes a voiced-unvoiced (V/U) decision, and, during periods of voiced speech, provides a measurement of the pitch period. However, some pitch detection algorithms just determine the pitch during voiced segments of speech and rely on some other technique for the voiced-unvoiced decision. Cf. IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-24, No. 5, October 1976, pp 399-418.
Several voiced-unvoiced detection algorithms are described in said last publication, based on the autocorrelation function, a zero-crossing count, a pattern recognition technique using a training set, or based on the degree of agreement among several pitch detectors. These detection algorithms use as input the time domain or frequency domain data of the speech signal in practically the whole speech band, while for pitch detection on the contrary the data of a low pass filtered speech signal are generally used.